When it comes to synonyms for the phrase "not trustworthy," you have a few options to choose from. Some of the most common ones include "unreliable," "untrustworthy," "deceitful," "dishonest," and "suspect." Each of these words indicates a lack of dependability or a fundamental dishonesty that makes someone or something difficult to rely on or place confidence in. Other possible synonyms include "shady," "suspicious," "doubtful," "questionable," and "devious," among others. Ultimately, the choice of words will depend on the specific context, tone, and emphasis that you want to convey.